The present invention relates to a stud unit and a skid-proof tire having the stud units for preventing vehicles from slipping on snow- or ice-covered roads.
Conventionally, snow tires having multiple studs on the outer circumferential face are used as skid-proof tires.
However, studs of the snow tires grind surfaces of roads when vehicles having the snow tires run on a road whose surface is dry and exposed. Powdered dust, which is worn from roads by grinding with studs of the snow tires, is blown up in the atmosphere and causes air pollution. The powdered dust is harmful for our health because it includes such harmful heavy metals as cadmium, lead, and the like. Further, traffic signs painted on roads are worn off by studs of the snow tires, which will be a factor in traffic accidents, and a heavy expenditure for repairing the road signs will be required.
To avoid the above noted disadvantages, recently, studless-tires are being used. The studless-tire is a type of snow tire. It is made of specially composed rubber which does not become hard even at low temperature and has multiple narrow grooves on the outer circumferential face thereof so as to increase contact area with the road and to increase friction therebetween. Note that the friction of the studless-tire with the road is not less than that of conventional snow tires having studs when the temperature is quite low; the friction of the studless-tire with the road decreases when the temperature comes close to 0.degree. C. The frictional function is undermined by ice or snow on the road. Mixing a low-temperature plasticizer with tire rubber to soften treads of tires at low temperature is know but this property continues for only about one year, after which the tread becomes quite hard.
As another means, melting snow and ice on roads by spraying calcium chloride and the like is attempted but roadside trees are blighted and secondary pollution, such as water pollution, occurs.
To solve these disadvantages, a skid-proof tire having stud units was disclosed in the Japanese Patent Provisional Publication (Kokai) Gazette No. 59-186704. Each stud unit thereof shown in FIG. 23 has a cylinder 2 and an elastic member 3 of such material as rubber therein. A stud 5 is passed through a hole 3a, which is bored in the center of the elastic member 3. The rear end of the stud 5 is engaged with an inside face of rear wall 2a of the cylinder 2. There is formed a large-neck section 6 at the front end of the stud 5, and the large-neck section 6 projects forward from the front opening of the cylinder 2.
In operating a vehicle having tires with many studs 5 on a road whose surface is clear, when the brakes are applied each stud 5 is inclined and the large-neck section 6 engages the front end wall of the cylinder 2 to prevent the stud 5 from being pushed inwards, whereby the front end of each stud 5 projects from the tread face of the tires until the vehicle stops, so that the surface of the road is substantially ground.
It is necessary that the diameter of the shaft of the stud 5 be large because a great force is applied to the stud 5 in the direction of its inclination when the vehicle accelerates or is braked. But, if the shaft of the stud 5 is larger, the elastic member 3 and the cylinder 2 must be larger, so that each stud unit must be heavy.
When the stud 5 is inclined, the elastic member 3 is sometimes clipped as in a jaw between the underside of the large-neck section 6 and the front end wall of the cylinder 2.
Because the durability and stability of the elastic member are low, the elastic member is apt to be permanently deformed by frequent inclination of the stud 5, so that the stud 5 sometimes will not return to the center.
Further, studs made of shape-memory alloy have been proposed so as to project and to retract by themselves but they have problems about stable function, cost, and durability, so that they have not been realized.